/Engine/Source/Runtime/Net/Core/Classes/Net/Serialization/FastArraySerializer.h
ReplicationID and ReplicationKeys are set by the MarkItemDirty function onFFastArraySerializer. These are just int32s that are assigned in order as things change. There is nothing special about them other than being unique. Helper to get auto-deduced FastArrayItemType from FastArraySerializer
template<typename FastArrayType> class TFastArrayTypeHelper